The invention is in the field of magnetic resonance imaging apparatus and methods and relates more particularly to the simultaneous imaging of a plurality of objects within a common magnet.
For the past twenty years, magnetic resonance phenomena has been employed to obtain noninvasive imaging of the interior of a body. Much of this effort has been directed to medical investigation of humans and of animals, in the form of structural investigations as well as spatially selective chemical shift studies. Spatial resolution can be extremely high (microscopic imaging at the cellular level is achievable). MRI is an attractive imaging method for medical purposes because it is unparalleled for imaging soft tissue and pathology. Other applications include the non-destructive investigation of geologic cores, eggs and the like.
Practical methods for these investigations require a complex sequence of excitation and encoding of spatial information in RF phase dependence. Image data is necessarily voluminous and the acquisition of such numerous data consumes a concomitant time interval. This limits the productivity, or throughput of a facility when screening of multiple samples is necessary. The time interval per object may be shortened with the sacrifice of spatial resolution and/or tolerance of greater noise in the data. Utilization of an MRI facility for concurrent examination of multiple objects would advance screening and commercial applications suited to study objects appropriate to the dimensions of the imaging volume of such apparatus.
A system employing multiple RF coils for acquiring signals from the same tissue is disclosed in U.S. Pat. No. 4,857,846 to Carlson. The plural coils are there coaxially disposed and comprise the signal sources for respective plural RF receiver channels. A reconstruction algorithm is employed to operate upon the simultaneously acquired signals (having respective phase dependence corresponding to spatial relationship of the coils to the object/patient) to produce an image of the interior of the object/patient. In this arrangement, for example, after excitation, a first receiving coil provides slice selected and y-phase encoded data while the second receiving coil provides slice selected and x-phase encoded data. The total time for acquiring an image can thus be reduced by about one half.
Efforts directed to obtaining chemical shift spectral data simultaneously from multiple samples is described by Fisher, et al, in J. Mag. Res., v. 138, pp160-163 (1999). A multi-sample probe is there disclosed comprising two RF coils, each coupled to respective samples and also coupled to respective receivers. This work suggests expanded capacity through completely parallel channels, including a receiver for each RF coil. Use of a single receiver through a multiplex arrangement with frequency encoding from gradient pulses is also described. The provision of multiple receivers on a one-to-one basis for each RF coil becomes expensive as the number of RF coils increases.
Concurrent chemical analysis of separate samples is also disclosed in U.S. Pat. No. 4,654,592 to Zens.
The acquisition of NMR data from multiple objects using a single receiver and multiple acquisitions with and without applied gradients is suggested by MacNamara, et al, Analytica Chemica Acta, v.397,pp.9-16 (1999). These techniques appear particular to chemical analysis and without applicablity to simultaneous imaging of multiple objects.
Instrumentation has been disclosed for the purpose of mapping a magnetic field incident to compensating undesired gradients therein. For this usage, a plurality of discrete samples are disposed at respective locii. These plural samples are represented as being coupled to respective receiver channels. However, details of this arrangement are not elaborated. See U.S. Pat. Nos. 4,949,043; 4,862,087.
The concurrent imaging of a number of separate objects is desirable for enhancement of throughput. The necessity of separate RF excitation apparatus in close proximity presents the occurrence of crosstalk and various artifact invited by undesired coupling of the separate units, or RF cells. Crosstalk between separate transmit and receive coils has been addressed in the practice of medical MRI apparatus. In that context it is often the case that a large body coil provides RF excitation to the body under study while smaller receive coils are deployed in one or more regions of interest. Both the transmit coil and receive coil(s) couple strongly to the body under study with the consequence that there is commonly found to be significant coupling between the transmit and receive coils. It is known to address this problem by providing for the detuning of one of these coils when the other such coil is active. Representative references are U.S. Pat. Nos. 4,810,968; 5,278,505; Mellor and Checkley, MAGMA, v. 3, pp. 35-40 (1995).
In the practice of medical MRI, it is also known to efficiently acquire multiple images of respective slices of the body under study by interleaving certain process steps of the resonant preparation and readout of the signals from respective slices. See: U.S. Pat. No. 4,318,043; Crooks L.E., Radiology, v. 153, pp. 459-65 (1984).
The present work is directed to the acquisition of images of multiple objects, and particularly objects of irregular shape, e.g., mice. This presents a challenge in both the equipment and the time required for the acquisition of the several images. The specific application requires the anatomical survey of large numbers of transgenic mice with mutated genes in order to study their morphologies. A typical screening might require the examination of 5000 mice per year and the images should exhibit spatial resolution of 50 xcexcm in each spatial coordinate. Using a fast spin echo pulse sequence for imaging, a data matrix of 700xc3x97700xc3x972000 would need to be acquired. Limiting the highest frequencies of a k space arrangement would serve to reduce the number of data points to xcfx80(700/2)2* 2000. Estimated time for a single such image acquisition is about 3 hours. Operation of the equipment on a continuous basis might require 625 days for the examination of 5000 mice. A need is therefore presented for the efficient, precise and simultaneous acquisition of nominally similar objects.
In a preferred embodiment of the present invention, the polarizing magnet, DC shims and pulsed gradient system of a magnetic resonance imaging (MRI) apparatus create a common magnetic environment for an array of plural RF coils with associated transmit/receive (TIR) switches, shields, and the like, arranged in sub-arrays. Each element of the array comprises a coil for independent imaging of the associated object. Grouping of N coils to form a sub-array allows for a manageable sharing of the modulated RF power for concurrent excitation of the N elements (N greater than 1) of that sub-array. The several (e.g., K greater than 1) sub-arrays are separately selected for excitation. For concurrent processing of the resonant signals from each object there are required N receiver channels which are selectively switched among the corresponding RF coils of each sub-array. The pulse sequence executed for acquiring the images is applied concurrently to the N RF coils of a subgroup and the sequence is interleaved among the subgroups. In the most straightforward description, each subgroup represents a common planar region of excitation for each of the N objects. Frequency encoding and phase encoding gradient pulses, in the well known art of magnetic resonance imaging, provide a parameter representing the density of resonant nuclei in the selected plane, or volume of the macro-object under study, e.g., the N objects. While N of the Nxc3x97K objects are imaged with a literal or absolute simultaneity in a specific portion of k-space, the several (K) sub-arrays are cyclically subject to the same partial image acquisition. Thus the entire k-space for each of the Nxc3x97K objects is acquired with substantial concurrency. The isolation of the several RF coils is improved by addressing the Kxc3x97Nxe2x88x92N coils in such a manner as to detune those RF coils which are instantaneously inactive. In the example described below, a floating conductor forming an (open) inductive loop is disposed the associated RF coil. This detuning loop is normally open, supporting no circulating RF current. A diode is disposed to close the loop when the diode is activated, thereby producing a coupled inductance to shift the resonant parameters of the RF coil. Thus the coupling to the RF coil shifts the resonant frequency away from the resonant frequency of the proximate active coil. Still further post-acquisition processing may be applied to remove artifacts from the several images.